1. Field of the Invention
The present invention relates to a constant current generation circuit for generating a constant current, a constant voltage generation circuit for generating a constant voltage, a constant voltage/constant current generation circuit for generating a constant voltage and a constant current, and an amplification circuit using the same.
2. Description of the Background Art
Reference current generation circuits for generating constant reference currents and reference voltage generation circuits for generating constant reference voltages are used for various analog circuits. In ALPC (Auto Laser Power Control) circuits and A/D (Analog-to-Digital) converters for CD (Compact Disk) drives, for example, constant voltage generation circuits for generating constant reference voltages which do not depend on the variation in power supply voltage, the temperature change, and the variation in processes are required.
On the other hand, frequency characteristics of operational amplifiers greatly depend on bias currents. If the bias currents are constant, the dependency on the variation in power supply voltage, the temperature change, and the variation in processes can be reduced, thereby making it possible to realize high-performance analog circuits. From such a point of view, constant current generation circuits are important in order to supply constant bias currents.
In recent years, the above-mentioned analog circuits such as the ALPC circuits, the A/D converters, and the operational amplifiers have been made one chip using the CMOS (Complementary Metal-Oxide Semiconductor) process. In this case, the constant voltage generation circuits and the constant current generation circuits must be designed by CMOS circuits.
Currents generated by the constant current generation circuits using the CMOS circuits vary by the variation in power supply voltage, the temperature change, and the variation in processes. The amount of the variation in this case is significantly large.
FIG. 8 is a circuit diagram showing an example of a conventional constant current generation circuit.
The constant current generation circuit shown in FIG. 8 is constituted by p-channel MOS field effect transistors 81, 82, and 87, n-channel MOS field effect transistors 83, 84, 85, and 86, and a resistor 88.
The transistor 81 has its source connected to a power supply terminal receiving a power supply voltage, has its drain connected to a node N81, and has its gate connected to a node N82. The transistor 82 has its source connected to the power supply terminal, and has its drain and its gate connected to the node N82. The transistor 83 has its drain connected to the node N81, has its source connected to a node N83, and has its gate connected to a node N84. The transistor 84 has its drain connected to the node N82, has its source connected to the node N84, and has its gate connected to the node N81.
The transistor 85 has its drain connected to the node N83, has its source connected to a ground terminal, and has its gate fed with an inverted stand-by signal STB. The transistor 86 has its drain connected to the node N84 through the resistor 88, has its source connected to the ground terminal, and has its gate fed with the inverted stand-by signal STB. The transistor 87 has its source connected to the power supply terminal, has its gate connected to the node N82, and has its drain supplied with a current IC.
The transistors 81 and 82 constitute a current mirror circuit, and a current which is equal or proportional to a current flowing through the transistor 81 flows through the transistor 82.
In the constant current generation circuit shown in FIG. 8, when the inverted stand-by signal STB enters a high level, the transistors 85 and 86 are turned on. Consequently, a current Ir flows from the power supply terminal to the ground terminal through the transistors 82 and 84, the resistor 88, and the transistor 86.
A current It which is equal or proportional to the current Ir flows from the power supply terminal to the ground terminal through the transistors 81, 83, and 85. In this case, a voltage applied across both ends of the resistor 88 is uniquely determined by a gate-source voltage of the transistor 83. Consequently, a constant voltage is applied across both ends of the resistor 88 irrespective of the power supply voltage. Therefore, the current Ir flowing through the resistor 88 does not depend on the variation in the power supply voltage.
In this case, the current Ir flowing through the resistor 88 is determined by the following equation:
Ir=Va/R=xcex2xc2x7(Vaxe2x88x92Vt)2 xe2x80x83xe2x80x83(A1)
Here, Va denotes a voltage applied across both ends of the resistor 88, that is, the gate-source voltage of the transistor 83, Vt denotes a threshold voltage of the transistor 83, and R denotes the resistance value of the resistor 88. Further, xcex2 is expressed by the following equation:
xcex2=(xc2xd)xc2x7(W/L)xc2x7Coxxc2x7xcexcxe2x80x83xe2x80x83(A2)
In the foregoing equation (A2), W denotes the gate width of the transistor 83, L denotes the gate length of the transistor 83, Cox denotes the capacitance of a unit oxide film of the transistor 83, and xcexc denotes the mobility of electrons or holes.
Conventionally, a bias voltage has been set such that the gate-source voltage of the transistor 83 is approximately equal to the threshold voltage Vt.
As described in the foregoing, in the constant current generation circuit shown in FIG. 8, the current IC is constant without depending on the variation in the power supply voltage. However, xcex2, Vt, and R in the foregoing equation (A2) vary depending on the variation in processes, and the current Ir and the voltage Va also vary depending on the temperature change. Consequently, it is impossible to obtain a constant current which does not depend on the temperature change and the variation in processes.
When a constant voltage generation circuit for generating a constant voltage is constructed using a CMOS circuit, a constant current generated by the constant current generation circuit is generally converted into a constant voltage using a resistance load. When the constant voltage generation circuit is constructed using the constant current generation circuit shown in FIG. 8, the current IC is converted into a voltage using the resistor. Also in this case, the current IC varies by the temperature change and the variation in processes. Accordingly, it is impossible to obtain a constant voltage which does not depend on the temperature change and the variation in processes.
An object of the present invention is to provide a constant current generation circuit composed of a field effect transistor and capable of generating a constant current without depending on the variation in power supply voltage and the temperature change.
Another object of the present invention is to provide a constant current generation circuit composed of a field effect transistor and capable of generating a constant current without depending on the variation in power supply voltage, the temperature change, and the variation in processes.
Still another object of the present invention is to provide a constant voltage generation circuit composed of a field effect transistor and capable of generating a constant voltage without depending on the variation in power supply voltage, the temperature change, and the variation in processes.
A further object of the present invention is to provide a constant voltage/constant current generation circuit composed of a field effect transistor and capable of generating a constant current and a constant voltage without depending on the variation in power supply voltage, the temperature change, and the variation in processes and an amplification circuit using the same.
A constant current generation circuit according to an aspect of the present invention comprises a first field effect transistor having a threshold voltage Vt; and a first resistor, the first field effect transistor and the first resistor being connected to each other such that the first field effect transistor operates in a saturation region, a voltage applied across both ends of the first resistor is uniquely determined by a gate-source voltage of the first field effect transistor, and a current flowing through the first field effect transistor and a current flowing through the first resistor are equal or proportional to each other, and the gate-source voltage of the first field effect transistor being set within a range of not less than (Vt+0.1) volts nor more than (Vt+0.4) volts.
In the constant current generation circuit, the first field effect transistor operates in the saturation region, and the voltage applied across both ends of the first resistor is uniquely determined by the gate-source voltage of the first field effect transistor. Accordingly, the voltage applied across both ends of the first resistor does not depend on the variation in power supply voltage. Further, the gate-source voltage of the first field effect transistor is set within a range of not less than (Vt+0.1) volts nor more than (V+0.4) volts, so that the voltage applied across both ends of the first resistor does not depend on the temperature change. Consequently, a constant current can be generated without depending on the variation in power supply voltage and the temperature change.
The constant current generation circuit may further comprise a first current mirror circuit for respectively causing currents which are equal or proportional to each other to flow through the first field effect transistor and the first resistor.
In this case, the currents which are equal or proportional to each other are respectively caused to flow through the first field effect transistor and the first resistor by the first current mirror circuit.
The constant current generation circuit may further comprise a second field effect transistor. The first current mirror circuit may comprise third and fourth field effect transistors. The first field effect transistor may have its gate electrically connected to one end of the resistor, have its source electrically connected to the other end of the resistor, and have its drain electrically connected to the drain of the third field effect transistor, the second field effect transistor may have its gate electrically connected to the drain of the first field effect transistor, have its source electrically connected to the one end of the resistor, and have its drain electrically connected to the drain of the fourth field effect transistor, the third field effect transistor may have its source electrically connected to a predetermined potential, and have its gate electrically connected to the gate and the drain of the fourth field effect transistor, and the fourth field effect transistor may have its source electrically connected to the predetermined potential.
In this case, when a current follows through the third field effect transistor and the first field effect transistor, a current which is equal or proportional to the current flowing through the first field effect transistor flows through the fourth field effect transistor, the second field effect transistor, and the first resistor. Particularly, the first field effect transistor operates in the saturation region, and the first resistor is electrically connected between the gate and the source of the first field effect transistor. Accordingly, a voltage applied across both ends of the first resistor is uniquely determined by the gate-source voltage of the first field effect transistor.
The first, second, third and fourth field effect transistors may be metal oxide semiconductor field effect transistors (MOSFETs).
The constant current generation circuit may further comprise potential holding means for holding the drain of the first field effect transistor at a predetermined potential. In this case, the drain of the first field effect transistor is prevented from being stabilized at an undesired potential.
The resistance value of the first resistor may be adjustable at the time of at least the fabrication. Even when the characteristics of the first field effect transistor vary, therefore, the resistance value of the first resistor is adjusted, thereby making it possible to set the gate-source voltage of the first field effect transistor within a range of not less than (Vt+0.1) volts nor more than (Vt+0.4) volts.
In this case, a maker can adjust the resistance value, and a user who has purchased a product having the constant current generation circuit can also adjust the resistance value.
The first resistor may be composed of polycrystalline silicon. Consequently, the temperature coefficient of the first resistor can be reduced, thereby making it possible to obtain a constant current which does not depend on the temperature change. Further, the first resistor may be composed of two-layer polycrystalline silicon. Consequently, the temperature coefficient can be further reduced.
The gate length and the gate width of the first field effect transistor may be set such that the voltage applied across both ends of the first resistor at a first temperature and a voltage applied across both ends of the first resistor at a second temperature different from the first temperature are equal to each other.
Consequently, the voltage applied across the first resistor is made constant without depending on the temperature change between the first temperature and the second temperature. As a result, a constant current which does not depend on the power supply voltage can be obtained.
The first resistor may be constructed using a plurality of resistors and a switch, and may have a programmable function by switching the plurality of resistors using the switch.
A constant voltage generation circuit according to another aspect of the present invention comprises a constant current generation circuit; and a current/voltage conversion circuit for converting a current generated by the constant current generation circuit into a voltage, the constant current generation circuit comprising a first field effect transistor having a threshold voltage Vt, and a first resistor, the first field effect transistor and the first resistor being connected to each other such that the first field effect transistor operates in a saturation region, a voltage applied across both ends of the first resistor is uniquely determined by a gate-source voltage of the first field effect transistor, and a current flowing through the first field effect transistor and a current flowing through the first resistor are equal or proportional to each other, the gate-source voltage of the first field effect transistor being set within a range of not less than (Vt+0.1) volts nor more than (Vt+0.4) volts, and the current/voltage conversion circuit comprising a second resistor composed of the same material as that for the first resistor in the constant current generation circuit, and a second current mirror circuit for causing a current which is equal or proportional to a current flowing through the first resistor in the constant current generation circuit.
In the constant voltage generation circuit, the current which is equal or proportional to the current flowing through the first resistor in the constant current generation circuit flows through the second resistor by the second current mirror circuit. Consequently, the current is converted into the voltage. In this case, the current flowing through the first resistor in the constant current generation circuit is made constant without depending on the variation in power supply voltage and the temperature change. Accordingly, a constant voltage is generated at both ends of the second resistor without depending on the variation in power supply voltage and the temperature change.
The second resistor is composed of the same material as that for the first resistor. When the resistance value of the first resistor varies on processes, therefore, the resistance value of the second resistor similarly varies. When the current flowing through the first resistor in the constant current generation circuit varies by the variation in the resistance value of the first resistor, therefore, the variation in the voltage generated at both ends of the second resistor in the current/voltage conversion circuit can be offset by the variation in the resistance value of the second resistor. Consequently, a constant voltage can be generated without depending on the variation in processes.
The resistance value of the second resistor may be adjustable at the time of at least the fabrication. When the output voltage varies, therefore, the voltage generated at both ends of the second resistor can be set to a desired voltage by adjusting the resistance value of the second resistor.
In this case, a maker can adjust the resistance value, and a user who has purchased a product having the constant current generation circuit can also adjust the resistance value.
The constant current generation circuit may further comprise a first current mirror circuit for respectively causing currents which are equal or proportional to each other to flow through the first field effect transistor and the first resistor.
In this case, the currents which are equal or proportional to each other are respectively caused to flow through the first field effect transistor and the first resistor by the first current mirror circuit.
The constant current generation circuit may further comprise a second field effect transistor. The first current mirror circuit may comprise third and fourth field effect transistors. The first field effect transistor may have its gate electrically connected to one end of the resistor, have its source electrically connected to the other end of the resistor, and have its drain electrically connected to the third field effect transistor, the second field effect transistor may have its gate electrically connected to the drain of the first field effect transistor, have its source electrically connected to the one end of the resistor, and have its drain electrically connected to the drain of the fourth field effect transistor, the third field effect transistor may have its source electrically connected to a predetermined potential, and have its gate electrically connected to the gate and the drain of the fourth field effect transistor, and the fourth field effect transistor may have its source electrically connected to the predetermined potential.
In this case, when a current flows through the third field effect transistor and the first field effect transistor, a current which is equal or proportional to the current flowing through the first field effect transistor flows through the fourth field effect transistor, the second field effect transistor, and the first resistor. Particularly, the first field effect transistor operates in a saturation region, and the first resistor is electrically connected between the gate and the source of the first field effect transistor. Accordingly, the voltage applied across both ends of the first resistor is uniquely determined by the gate-source voltage of the first field effect transistor.
The first, second, third and fourth field effect transistors may be metal oxide semiconductor field effect transistors.
The constant current generation circuit may further comprise potential holding means for holding the drain of the first field effect transistor at a predetermined potential. In this case, the drain of the first field effect transistor is prevented from being stabilized at an undesired potential.
The resistance value of the first resistor may be adjustable at the time of at least the fabrication. When the characteristics of the first field effect transistor vary, therefore, the gate-source voltage of the first field effect transistor can be set within a range of not less than (Vt+0.1) volts nor more than (Vt+0.4) volts by adjusting the resistance value of the first resistor.
In this case, a maker can adjust the resistance value, and a user who has purchased a product having the constant current generation circuit can also adjust the resistance value.
The first resistor may be composed of polycrystalline silicon. Consequently, the temperature coefficient of the first resistor can be reduced, thereby making it possible to obtain a constant current which does not depend on the temperature change. Further, the first resistor may be composed of two-layer polycrystalline silicon. Consequently, the temperature coefficient can be further reduced.
The gate length and the gate width of the first field effect transistor may be set such that a voltage applied across both ends of the first resistor at a first temperature and a voltage applied across both ends of the first resistor at a second temperature different from the first temperature are equal to each other.
Consequently, the voltage applied across the first resistor is made constant without depending on the temperature change between the first temperature and the second temperature. As a result, a constant current which does not depend on the power supply voltage can be obtained.
The second resistor may be constructed using a plurality of resistors and a switch, and may have a programmable function by switching the plurality of resistors using the switch.
The first resistor may be constructed using a plurality of resistors and a switch, and may have a programmable function by switching the plurality of resistors using the switch.
A constant voltage/constant constant current generation circuit according to still another aspect of the present invention comprises a constant voltage generation circuit, the constant voltage generation circuit comprising a constant current generation circuit, and a current/voltage conversion circuit for converting a current generated by the constant current generation circuit into a voltage, the constant current generation circuit comprising a first field effect transistor having a threshold voltage Vt, and a first resistor, the first field effect transistor and the first resistor being connected to each other such that the first field effect transistor operates in a saturation region, a voltage applied across both ends of the first resistor is uniquely determined by a gate-source voltage of the first field effect transistor, and a current flowing through the first field effect transistor and a current flowing through the first resistor are equal or proportional to each other, the gate-source voltage of the first field effect transistor being set within a range of not less than (Vt+0.1) volts nor more than (Vt+0.4) volts, the current/voltage conversion circuit comprising a second resistor composed of the same material as that for the first resistor in the constant current generation circuit, and a second current mirror circuit for causing a current which is equal or proportional to the current flowing through the first resistor in the constant current generation circuit to flow through the second resistor, and the constant voltage/constant current generation circuit further comprising a third current mirror circuit for generating a current which is equal or proportion to the current flowing through the first resistor in the constant current generation circuit in the constant voltage generation circuit.
In the constant voltage/constant current generation circuit, a constant voltage and a constant current can be generated in a small area without depending on the variation in power supply voltage, the temperature change, and the variation in processes.
An amplification circuit according to a further aspect of the present invention comprises a plurality of operational amplifiers; and a constant voltage/constant current generation circuit for applying a constant voltage as a reference voltage to an input terminal of at least one of the plurality of operational amplifiers as well as supplying a constant current as a bias current, the constant voltage/constant current generation circuit comprising a constant voltage generation circuit, the constant voltage generation circuit comprising a constant current generation circuit, and a current/voltage conversion circuit for converting a current generated by the constant current generation circuit into a voltage, the constant current generation circuit comprising a first field effect transistor having a threshold voltage Vt, and a first resistor, the first field effect transistor and the first resistor being connected to each other such that the first field effect transistor operates in a saturation region, a voltage applied across both ends of the first resistor is uniquely determined by a gate-source voltage of the first field effect transistor, and a current flowing through the first field effect transistor and a current flowing through the first resistor are equal or proportional to each other, the gate-source voltage of the first field effect transistor being set within a range of not less than (Vt+0.1) volts nor more than (Vt+0.4) volts, the current/voltage conversion circuit comprising a second resistor composed of the same material as that for the first resistor in the constant current generation circuit, and a second current mirror circuit for causing a current which is equal or proportional to the current flowing through the first resistor in the constant current generation circuit to flow through the second resistor, and the constant voltage/constant current generation circuit further comprising a third current mirror circuit for generating a current which is equal or proportion to the current flowing through the first resistor in the constant current generation circuit in the constant voltage generation circuit.
In the amplification circuit according to the present invention, a constant voltage can be applied as a reference voltage to the input terminal of at least one of the plurality of operational amplifiers without depending on the variation in power supply voltage, the temperature change, and the variation in processes, and a constant current can be supplied as a bias current. Consequently, an amplification circuit which does not depend on the variation in power supply voltage, the temperature change, and the variation in processes is realized.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.